More Steps in Motorcycle Evolution

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

If you want to fully appreciate your bike, it helps to understand the technical path it traveled to get here.

Valve Trains Evolve

Early motorcycle engines often had the “pocket-valve” system of the de Dion-Bouton powerplants that were so influential around 1900. This began with an “automatic” inlet valve that was held closed by a light spring and not pushed open by a camshaft, but pulled open by the partial vacuum created as the piston descended on its intake stroke.

Because throttle response with automatic intake was sudden, beginning around 1903 some builders placed the intake valve adjacent to the exhaust and next to the cylinder, with both their stems pointing down. In this side-valve or “flathead” configuration the combustion chamber extended to one side of the piston, covering the valve heads. With all the valve mechanism now quite close to the oil storm in the crankcase, the assembly was not only well lubricated but could be easily enclosed, making the engine cleaner and protecting the valve train from dirt.

The side-valve arrangement considerably restricted engine airflow because mixture from the carburetor had to flow up and through the intake valve, then turn at right angles to flow across to the cylinder, then make another right-angle turn downward to fill it. But first things first. The fully enclosed side-valve design was a big step in civilizing the motorcycle and making it appeal to more people.

Bigger Brakes for Higher Speeds

From the experience of many riders with a variety of possible brake designs, the drum-and-pivoted-internal-shoe brake emerged as easy to make, with its friction surfaces somewhat protected from weather. Bicycle brakes were operated either by Bowden cable or articulated rod, so these systems, already in large-scale production, were naturally adopted on motorcycles.

The tiny drum brakes of the early 1920s—as small as 3 inches!—are a joke today, but we owe their steady improvement to the natural regime of design, ride, crash, and redesign; perhaps not always keeping up with increasing engine power, but still growing all the time. In thinking about design, always remember that rational thought is not the only force in play. People get used to things as they are, and when someone changes that, it can be upsetting. When Erik Buell put a wind-tunnel-proven low-drag fairing on one of his production bikes, many called it “the Whale” and thought it ugly. Liquid-cooled Harleys? Plenty of traditionalists are still trying to get their heads around that.

The Telescopic Fork

Whenever a novelty such as the motorcycle arrives, a rich variety of ideas is immediately applied to it. Although the girder fork was the dominant front suspension between 1920 and 1950, many alternatives were tried, among them telescopic designs. What really counts in design is not who invented the first crude version, but rather who designed something of function superior enough to justify the cost of tooling it for production. Lathes, mills, and centerless grinders cost molto bucks.

BMW engineers in the early 1930s saw that a telescopic fork without internal lubrication and workable seals was bound to quickly grind itself useless. They also had the sense to reject the jerky action of dry friction dampers in favor of smooth hydraulic damping. Where in the patent records could such dampers be found? In the recoil systems of artillery! By 1945 BMW had worked through the major damper types now being used in 21st-century bikes: damper-rod, cartridge, and through-rod. They started with small, springy fork tubes and went larger until, with the fork on the military R75 sidecar rig, they reached a high standard. Postwar builders in the US and Britain acquired R75s for study.

Overhead Valves (OHV) Dominate

The mechanism for operating intake and exhaust valves evolved rapidly as the IC engine left the category of novelty and demonstrated its usefulness. Giving side-valve designs oil-tight enclosures was easy, but the higher performance overhead-valve engines being adopted right after World War I posed another problem: Enclosing and lubricating the mechanism without obstructing the flow of cooling air to cylinder-head fins.

The performance advantage of the OHV’s straighter flow path was too great to let such matters stand in the way. Rocker arms were bracketed above the head, with exposed pushrods running up from tappets protruding through the top of the engine’s timing case. No lubrication at all! Full enclosure would take time.

Improved Valve-Train Lubrication

More cogent was the failure of cams and followers under the doubled spring pressure found necessary to control the OHV’s heavier valve train. Hmmm. Maybe timing cases could not, after all, be lubricated by accident or “oil vapor.” Maybe it would be a good idea to route the oil, on its way from oil tank to crankcase, up through the timing case? And definitely, the early schemes for making a single cam lobe operate both intake and exhaust valves would have to go; under-lubrication wiped them out very quickly with OHV spring pressures.

One solution was to provide a separate cam and follower for each valve, and in England AJS did just this—at the very time that executives from Harley-Davidson went on a world tour seeking new ideas. Very quickly (in the form of its 350 “Peashooter” singles) H-D adopted the idea, moving it to a V-twin by 1929, where it lives on to this day (or, at least until a few of months ago) in the four separate geared cam wheels of the Sportster.

Pumped Recirculating Oil Systems

Motorcycling long resisted the inevitable future of pumped, recirculating oil systems, preferring to rely on piston-driven crankcase airflow or other comic half measures to keep balmy oil between moving parts.

When the Goodman family decided in the mid-1920s that Velocette must offer a four-stroke, they incorporated the latest technologies seen in the avant-garde specials running at Brooklands. Great numbers of overhead-cam aircraft engines had also been built for the Great War, and their example was widely known. Velo’s engine was therefore given a pumped, recirculating oil system and an overhead-cam valve train. In addition, Velo found that circulating oil through an engine leveled its temperatures, cooling hot regions and warming cool ones. Lubricating the cam and its followers, remote though they were from the oily crankcase, was now just a matter of sending pumped oil up to the head and then returning it.

Foot Shift Arrives

The Isle of Man TT course wasn’t fully paved until 1926. Previously, gear-changing with a three-speed box required taking a hand off the bar and moving a long shift lever among its four gated positions. As motor racing moved away from jolting over ruts and toward fluid grace, the interruption imposed by such gear-changing became more costly. Various schemes were tried to substitute foot for hand (thereby making steering control continuous), but not until Velo’s race engineer Harold Willis’ ratchet-driven sequential shifter of 1927 did gear-changing take its present form: A rotary drum or plate with cam slots controlling the positions of sliding gears and held by a detent in any one of N + 1 positions (where N is the number of speeds), and moved from one to the next by a ratchet that allowed the foot shift pedal to return always to its center position.

Suppressing Knock With Squish

Moto-historians marvel that Charles Franklin, Indian’s chief engineer (c. 1917–1931), was able to build side-valve engines that often defeated supposedly more advanced OHVs in American dirt-track racing. Franklin wasn’t telling, but he was one of at least four engineers who independently recognized the anti-knock benefit of “squish.”

Knock or detonation severely limited the power of early engines. Raise compression too high in an effort to boost torque and deto’s hard little teeth would start gnawing on your pistons. Twenty years of chemistry would shift the odds in our favor, but early fuels knocked easily, even at low compression ratios of four or five to one.

Knock needs time, so it occurs most easily at low engine speeds (lugging) and in engines with slow combustion. Squish is a means of stirring up the fuel-air mixture and making it burn faster, outrunning detonation. Squish is any area of piston top that is made to come very close to a matching area of the cylinder head at top dead center (TDC). The mixture between the two surfaces is “squished” rapidly outward, forming a jet that stirs the charge. Charles Franklin’s “secret” was to give his side-valve “PowerPlus” race engines enough squish area to let him raise their compression ratio higher than was safely knock-free in the OHV competition. Put in simplest terms, Franklin’s squish-enabled Indians could run higher compression than the opposition, regardless of the valve layout. Yes, the OHVs like the Indian and Harley “Eight Valves” flowed better, and so maybe made more power on top, but riders on squish Indians could take a handful of throttle at low revs and accelerate harder thanks to their higher compression ratio.

Why couldn’t OHV engines use squish too? When compression was five to one, combustion chambers were so big that no part of an OHV piston came anywhere near the head. But in a side-valve, most of the chamber volume was offset over the valves, so a part of the head directly over the piston could be brought down close to the crown.

Later on, when more knock-resistant fuels made 10-to-1 compression workable, OHV/OHC pistons had come close enough to the head to make squish workable for them too. Norton introduced squish pistons on its factory roadracers in 1950.

Improving Engine Airflow

Englishman Harry Weslake wasn’t the only one working on the engine airflow problem in the 1920s, but he told the story well. He bought three Sunbeam 500 race engines for a sponsored rider, but upon dyno-testing them, he found they made 25, 26, and 29 hp respectively. Why? All were outwardly identical, but the best made 16 percent more power than the worst.

Concluding that this must have something to do with the engines’ air-taking efficiency, he constructed a flow-measuring device and was able to show that there was wide variation in the airflow through valve ports, depending upon the accidents of casting-core shift and machining. At roughly the same time (early 1920s), similar work was ongoing at the US Army’s air development labs at McCook Field in Dayton, Ohio. (Interesting to note that Maldwyn Jones, one of the early American dirt-track star riders and innovators, was a dyno operator at McCook.)

Why even worry about port shape? Just hog those suckers out!

That was the old way, but it leaves out the cylinder-filling ability of high port velocity. As the piston slows to a stop at bottom center, intake flow is still moving at near the speed of sound, and it will keep right on coasting into the cylinder as long as the intake valves remain open. But with a huge “hogged out” port, flow velocity will be much lower, and the rising piston will stop and reverse its flow long before it does in that of a smaller, streamlined high-velocity port.

Improved Gasolines = More Knock Resistance

How were fuels improved? Don’t all hydrocarbons give pretty much the same amount of heat when burned? True, but the better a fuel resists detonating, the higher the compression ratio you can use with it, and the more torque and power you can safely make—even though the knock-resistant fuel and Knock-O swill (you can almost hear it knocking in the can!) produce the same heat energy. In the high-compression engine, more heat is converted to pressure pushing pistons, while the lower-compression engine burning Knock-O sends that heat out its exhaust pipe.

Soon after 1920, GM’s Charles Kettering assigned Delco chemist Thomas Midgley the task of finding a fuel additive that suppressed knock. Using many test engines, he found that an extremely poisonous substance, tetraethyl lead (TEL), dramatically reduced knock even when used in amounts as small as 1 gram per gallon of gasoline. US aviation gasoline, treated with TEL, was crucial in the 1940 Battle of Britain, which pitted British Hurricane and Spitfire fighters against Hitler’s bomber force.

Replacing Lead in the Clean-Air Era

When it was decided to clean up auto exhaust to reduce urban smog in the 1960s, the exhaust-treatment catalysts that could do the job were inactivated by TEL, and consequently leaded pump gasoline was phased out in the later 1970s.

Today, the knock resistance once provided by Midgley’s TEL now comes from a combination of three developments:

1. Faster combustion resulting from high-velocity port flow and combustion-chamber shape.<br/>
2. Gasolines formulated with more knock-resisting feedstocks such as aromatics, alkylate, and ethyl alcohol.<br/>
3. Accurate digital control of spark timing and fuel delivery, with direct knock suppression triggered by knock sensors.<br/>

The English Path vs. the Continental Way

It’s curious that in England, motorcycle performance was chiefly enhanced in the period between 1920–1950 by meticulous improvements to very basic single-cylinder 350cc and 500cc engines whose structure limited them to about 7,000 rpm. Most work went into refined flow, combustion, and cooling improvements.

Meanwhile, in Italy, Benelli in the 1930s found it could boost the power of its 250 by making it breathe at higher revs. After WWII, Germany’s NSU explored revs up to 12,000 with its 125 singles. Postwar Italian high-revvers included the 13,000-rpm Mondial 125 and Guzzi’s 12,000-rpm 500cc V8.

The “British way” was merged into the development of F1 auto racing engines in the late 1950s and ‘60s, surviving to this day in the form of respected specialist firms such as Cosworth, Ilmor, and others.

In Japan, the high-rpm path of Germany and Italy was chosen by Honda in the mid-1950s, leading to its long list of motorcycle Grand Prix successes of the 1960s. Its final five-cylinder 125 revved to 21,000 and its research extended to 27,000.

Rear Suspension for Motorcycles

Comfort is not the only reason road vehicles have movable wheel suspension. When hitting a bump on a hardtail bike tosses the back end into the air, all wheel grip is briefly lost. If the vehicle is going around a turn at the time, the wheel tossed into the air “steps out”—jumping sideways because its tire is out of contact with the pavement. That directly limited the corner speed of bikes on turns less smooth than a billiard table. The harder you rode the more your bike stepped out, forcing you to limit your pace or run off the road. Only the acceleration of gravity—1 G—was available to stop that wheel’s upward movement, reverse it, and return it to its job of providing grip.

Now consider the situation when we provide suspension: Let’s say the front wheel carries a load of 250 pounds on our example bike, and that the wheel, tire, brake, and fork sliders together weigh 50 pounds. When a bump tosses the wheel upward, at what rate does it return to its duties? Acceleration is thrust (in this case, the 250 pounds with which the front springs press down against the wheel) divided by the weight of 50 pounds: 250/50 = 5 G. So, with suspension, our wheel’s “air time” is greatly shortened by being accelerated back downward at five times the acceleration of the wheel in the no-suspension case. The more of the time we can keep both tires on the pavement, the faster we can corner.

That advantage is just what engineers at Guzzi were after in 1935 when they went against all the best advice (“Nothing steers like a rigid”) and gave the rear end of their 500 racer a swingarm suspension. Rider Stanley Woods defeated all the rigid bikes, forcing engineers everywhere to reconsider rear suspension. Swingarms are near-universal on motorcycles and scooters today.

That ratio between wheel weight and wheel load is what engineers are talking about when they refer to “unsprung weight ratio,” and is why designers work so hard to reduce the weight of the unsprung components: wheel, tire, brake, etc. As just a small example, the brake calipers Yamaha put on its potent TZ750A roadracer of 1974 weighed 4 pounds each. The Brembo calipers on today’s MotoGP bikes each weigh a single pound.

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